The present invention relates generally to integrated circuit packaging and, more particularly, to hermetically packaged electronic systems embodying a high density interconnect (HDI) structure.
Hermetically sealed packages have long been used to package semiconductor devices for use in high reliability systems and in hostile environments. A primary advantage of a hermetically sealed package for a sensitive electronic device or circuit is that the hermetic seal ensures that no outside contaminants or other deleterious materials can reach the sensitive device, component or circuit. This ensures that a device which, after hermetic sealing, meets specifications during testing, will not deteriorate as a result of contamination or the introduction of other deleterious materials. This assurance is particularly important in systems which must exhibit high reliability.
While production of a container for an electronic circuit which can be hermetically sealed does not seem, at first glance, to be a particularly complicated process, this initial perception can be misleading, especially when a new container configuration is required. Lead time for fabrication of a new container configuration normally runs at least 4-6 months and often more. If changes are required, the same period of time is needed for revisions. Consequently, when a new system is designed, it must either be designed to fit into an existing hermetic enclosure or substantial lead time must be provided for designing and fabricating the hermetic enclosure for that system.
As disclosed in Eichelberger et al. U.S. Pat. No. 4,783,695, and related patents, a high density interconnect (HDI) structure which has been developed by General Electric Company offers many advantages in the compact assembly of digital and other electronic systems. For example, an electronic system such as a microcomputer which incorporates between thirty and fifty integrated circuit chips, or even more, can be fully assembled and interconnected on a single HDI substrate which is fifty mm long, fifty mm wide and 1.27 mm thick.
This HDI structure can be disassembled for repair or replacement of a faulty component and then reassembled without significant risk to the good components incorporated within the system. Repairability is particularly important where fifty or more chips having a cost of as much as $2,000.00, each, may be incorporated in a single system on one substrate. This repairability is a substantial advance over prior interconnection systems in which reworking the system to replace damaged (i.e., faulty) components is either impossible or involves substantial risk to the undamaged (i.e., good) components. Exemplary testing and repair techniques are disclosed in Eichelberger et al. Pat. Nos. 4,878,991, 4,884,122 and 4,937,203.
Very briefly, in the manufacture of one embodiment of systems employing the aforementioned HDI structure, individual cavities (or one large cavity) having appropriate depths at the intended locations of the various chips are formed in a component-supporting surface of the ceramic substrate. The various integrated circuit chips and other components are placed in their desired locations within the cavities and adhesively attached to the substrate.
At this stage, the upper surfaces of all components and portions of the substrate component-supporting surface are disposed substantially in a common plane. A multi-layer high density interconnect (HDI) overcoat structure including interleaved layers of dielectric material and metallized conductive material is then built up to electrically interconnect the components into a functioning system.
The HDI overcoat structure typically does not extend all the way to the outer edge of the substrate component-supporting surface; rather, the overcoat structure terminates just inside a row of contact pads to which external connections are subsequently made, such as by ultrasonic wire bonding, when the system is finally assembled into a suitable leaded package. These contact pads are formed directly on portions of the substrate surface surrounding the cavities, and are electrically connected through suitable vias within the HDI overcoat structure to lower metallization layers of the HDI overcoat structure. Metallization deposited or otherwise formed directly on the ceramic substrate, as opposed to metallization layers within the HDI overcoat structure, is referred to as "metal zero".
This connection of the HDI electronic system into a separate leaded package, such as by conventional wire bonding, creates an additional interface, increases thermal impedance, and ultimately results in higher assembly costs than is achieved in accordance with the present invention.